Multiple optical channels

ABSTRACT

The invention relates to an electronic distance measuring apparatus for surveying, e.g. for measuring the distance from the apparatus to an object. The apparatus comprises: a) an objective lens ( 101 ), defining an optical axis (OA), b) at least two sources ( 111, 112 ) of structured light for transmitting beams (λ 1 , λ 2 ) of separate wavelengths towards said object, said beams on reflection from said object received by the objective lens ( 101 ), (c) at least two receivers ( 141, 142 ) arranged outside a beam path ( 109 ) as defined by the objective lens ( 101 ), and adapted to receive said received beams (λ 1 , λ 2 ) of structured light. Optical means comprising at least two dichroic surfaces ( 121   a,    122   a ) are each arranged at a tilt angle (α 1 , α 2 ) with respect to said axis (OA), said optical axis (OA) passing through said surfaces, at least one of said dichroic surfaces arranged on a plate ( 121, 122 ), said at least two dichroic surfaces ( 121   a,    122   a ) adapted to reflect at least one of said received structured light beams (λ 1 , λ 2 ), respectively, towards said receivers ( 141, 142 ).

FIELD OF THE INVENTION

The present invention generally relates to an automatic surveyinstrument comprising a source of structured light for transmitting abeam of said light towards said object, a receiver adapted to receivesaid reflected structured light beam when reflected from said object,said received light being co-axial with the transmitted light, anobjective lens and a reticle with optical means positionedthere-between, said objective lens and said reticle defining an opticalaxis. The invention more specifically relates to the arrangement forredirecting the reflected light in the receiver.

BACKGROUND ART

Automatic survey instruments of the present type comprise a telescopesystem where received light can be divide into separate channels forrange-finding, tracking and viewing or manual aiming. The transmitterbeam for the range-finder and for the tracker function are preferablycoaxial to the receiver optics.

The light reflected from the target object, and received at theinstrument is divided, depending upon the purpose, in light componentsof different wavelengths such as the light for tracking, range findingand visible light for viewing. By using the tracking light and rangefinding light thus divided, range finding and automatic tracking may beperformed.

Such instruments, using more than one channel, according to the priorart, uses different types of prisms in the receiver with or withoutdichroic coatings for splitting/separating the beams into the differentchannels.

The word dichroic in this context refers to dichroic mirrors/prisms orcoatings, which exhibit selective reflection and transmission of lightas a function of wave-length regardless of its plane of vibration. Adichroic mirror thus selectively reflects light according to itswave-length and transmits light having other wavelengths. The use ofprisms, i.e. cubic prisms primarily requires long building lengths,problems in providing needed filters within the prisms and inherentlyhigh costs, in that the costs for producing the prisms are high.

Belonging to the related art are e.g. EP-1 081 459 and EP-0 987 517. Inthese discussions of problems related to the use of prisms can be found.In EP-0 987 517 e.g. the arrangement includes several prisms in whichtwo beams having different wavelengths are separated using two dichroicprisms. In EP-1 081 459 prisms are disclosed, but also one dichroicplate is used for separating beams with different wavelengths; however,that plate is arranged perpendicular to the optical axis of theinstrument, thus reflecting the light in the coaxial direction, whileallowing other wavelengths to pass through the plate.

The use of prisms, however, introduces certain problems. Drawbacks arethe fact the glass extends the optical path so the building lengthincreases, the size of the prisms become large and the prisms are heavy.The requirements on parallelity between prisms surfaces are very highand making a dichroic filter within the glass is harder than on asurface between air and glass. All this means that the prisms becomelarge and expensive and the telescope becomes larger and heavier. Moreadvanced prism constructions can reduce building length but the costsfurther increases for providing the prisms and the complicated structureresulting therefrom. Aditionally, the prisms make the apparatus largerand heavier causing other problems as increased power consumption, aswhen automatically tracking an object.

Examples of wavelengths used for visible light are in the range of 400nm to 650 nm for collimating purposes. For range-finding visible lightat 660 nm or IR light at 850 nm can be used and for tracking IR light at785 nm. These wavelength ranges are generally termed channels of aparticular type such as visual channel, tracking channel etc.

DISCLOSURE OF THE INVENTION

The present invention intends to overcome the above mentioned problem bya solution which is both simple and inexpensive.

This object has been achieved by a automatic survey instrument of theinitially defined kind, wherein tilted dichroic plates arrangedprimarily on the optical axis defined by the objective lens and thereticle of the instrument, which according to this invention preferablyalso coincides with the visual channel, i.e. the optical path for thevisible light used when viewing the target through the apparatus.Secondary mirrors may be used to fold the optical paths within theapparatus. These secondary mirrors may be of the dichroic type.

Substituting prisms for separating the different channels with platesresults in simpler components, simpler coatings and generally shortertelescope length. As referred to above with the use of prisms, thechannels are separated and/or reflected by passage through the prism,entering the prism from air through a first surface and exciting theprism to air. During that process the light (IR or visual) is eitherreflected against second prism surface and thereafter exists the prismthrough a third surface of the prism or it is allowed to pass throughthe second surface. The resulting optical path within the prism mayeither be depending on the type of prism material used or in this casethe second prism surface may have a dichroic coating. Depending on thedesirability of short telescope lengths this will increase thecomplexity of the prisms.

According to the invention the prisms have been replaced by tilteddichroic plates and/or mirrors; however, one prism may be used in one ofthe optical paths. This results in simpler components, simpler coatings,lower costs, and shorter telescope lengths, as will be shown.

By using the tilted dichroic plates it is possible to reflect thedifferent channels in a direction where the reflected beam isnon-coaxial with the main optical axis of the apparatus and thus arrangefor receivers or sensors placed adjacent to a volume as defined by theaperture and the focusing lens. The plates will, however, introduceoptical aberrations in the beam passing through the plate, e. g. comaand astigmatism, as the plates by necessity have to have a certainthickness in order to be stable.

These effects may be compensated for in a number of ways, which will beshown in the different embodiments below.

Making the plates slightly wedge-shaped will compensate the aberrationsfrom the tilted plate. The wedge angle has to be chosen in relation tothe tilt angle and the thickness of the plate. A person skilled in theart may, using an optical design program, optimize the wedge angle.

A second method is to add a third plane-parallel compensating platetilted around an axis perpendicular to the tilt axes of the firstplates.

Tilting two plane parallel plates around axes that are perpendicular toeach other is a third manner in which to reduce the aberrationsintroduced by the tilt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows schematically a side view of a first embodiment accordingto the invention.

FIG. 1 b shows a detail of FIG. 1 a

FIG. 2 a shows schematically a side view of a second embodimentaccording to the invention.

FIG. 2 b shows schematically a detail of FIG. 2 a of the definition ofthe wedge angle.

FIG. 3 shows schematically a side view of a third embodiment accordingto the invention.

FIG. 4 a shows schematically a side view of a fourth embodimentaccording to the invention.

FIG. 4 b shows schematically a detail of FIG. 4 a in a top view.

FIG. 5 shows schematically a side view of a fifth embodiment accordingto the invention.

In the drawings like details are shown using corresponding referencenumbers.

DETAILED DESCRIPTION OF THE EXEMPLARY PREFERRED EMBODIMENTS

In FIG. 1 is shown a first preferred embodiment of an apparatusaccording to the invention in which an objective lens, 101 a and 101 b,defines an optical axis OA. On this axis is seen a focusing lens 104 anda reticle 105. The focusing lens 104 is movable as is known to the manskilled in the art for focusing of the incoming light on the reticle.The image may be viewed through an eyepiece 106. Adjacent to theobjective and preferably attached to the inside of the lens 101 b, aprism 102 with a reflective surface 103 is arranged on the optical axisOA. The surface 103 of the prism 102 may have a dichroic coating 103.The purpose of this prism is to reflect the beams λ₁ and λ₂ ofstructured light from two transmitters 111 and 112 in the form ofsources of structured light, such as a laser or a Light Emitting Diode(LED), forming a transmitted light path through the prism 102, where thelight is reflected out through the objective lens 101 towards a target.The prism shown is in the form of a 45°-90°-45° prism used as a“right-angle prism”, i.e. the prism will turn the beam through a90°-bend. This requires that the beams λ₁ and λ₂ arrive at the opticalaxis perpendicular to said axis. This set-up simplifies the geometry.However this perpendicularity is not necessarily an essential element ofthe invention.

Both beams λ₁ and λ₂ exit the objective lens 101 coaxially as shown inthe figure. In order to accomplish this, the transmitters have beenarranged such that the beams λ₁ and λ₂ each are reflected in arespective dichroic mirror 113 and 114, directly or via a light guide,such that the thus reflected beam incident on the mirrors 113 and 114form a coaxial beam hitting the prism 102, i. e. the beam λ₁ transmittedfrom the transmitter 111 is reflected in the mirror 113 and istransmitted through the mirror 114, while the beam λ₂ transmitted fromthe transmitter 112 is reflected in the mirror 114 thus forming a commonoptical path.

The light reflected from the target reaches the objective lens in theform of a beam composed of the transmitted wavelengths and normallybroadened as to cover the total area of the objective lens. It will beunderstood that the prism 102 is small in relation to the area of theobjective lens and thus will obstruct a relatively small part of thecomposed beam λ₁, λ₂ reflected by the target.

Two dichroic plates 121, 122 are arranged in a tilted manner in thereflected beam on the optical axis OA, between the prism 102 and thereticle 105. “Tilted” in this context should be taken to mean that theplates are arranged such as to not be perpendicular to the optical axisOA. Part of the beam having the wavelength λ₁ is reflected in the firsttilted plate 121 towards a mirror 131, which reflects the beam towards adetector 141. Likewise the part of the beam, having the wavelength λ₂ isallowed to pass through the first plate 121 on to tilted plate 122 whereit is reflected towards a second mirror 132 which reflects the beamtowards a detector 142. The plates being tilted and not perpendicular tothe beam. Also it is not required that the mirrors be perpendicular tothe beam as is shown in the figure. The mirrors must of course be placedsuch as to deflect the beam in the direction of the receiver.

The two receivers (141,142) are arranged outside a beam path (109) asdefined by the objective lens (101).

In this embodiment a further emitter 115 is indicated arrangedperpendicular to the optical axis, which may emit light of a wavelengthλ₃. This may be used for some third reason e.g. as a visible beam to aidpointing to a target.

The tilt angle α is shown in FIG. 1 b, which shows a detail of FIG. 1 a.The tilt angle is defined as the angle between the normal N₁, N₂ to theplate and the optical axis OA. The axis around which the plate is tiltede.g. the axis indicated as 150, defines a tilt direction will be shownbelow to be of interest.

In FIG. 2 a a second embodiment according to the invention is shown inwhich like numbers are used to indicate corresponding details. The lenssystem 201, the prism 202, the focusing lens 204, the reticle 205, theeyepiece 206 and the optical axis AO are similar to each other and serveessentially the same purposes.

In this second embodiment transmitter 211, transmitting light of thewavelength λ₁, is arranged as in the first embodiment. The secondtransmitter 212 transmitting light of the wavelength λ₂, is arranged inthe same manner as transmitter 115 in the first embodiment, making onedichroic mirror superfluous.

The optical path for λ₁ for the light reflected from the target willessentially be the same as in the first embodiment via the dichroicmirror 221 and mirror 231 towards the receiver 241. Further the tiltangle of the dichroic plates 221 and 222 have been changed such that theonce reflected light of the wavelength λ₂ is directed towards thereceiver 242. This has been accomplished by resizing the distancebetween the plate 221 and/or by changing the tilt angles of the platesto allow the beam λ₂ reflected in plate 222 to reach the receiverpassing essentially adjacent to plate 221, i.e. plate 221 will not reachin to the reflected beam from plate 222. This figure also illustratesthe capability to direct the beam to the detector without a secondreflecting mirror.

The dichroic plates may exhibit a wedge form in order to compensate foroptical aberrations in the visual channel. This is illustrated in FIG. 2b in which the two plates 221 and 222 are shown, and the optical axis OAis indicated. The wedge form of the plates defines an angle of 0-30 arcseconds and will compensate for optical aberrations. It should beunderstood that this type of compensation may be used in the embodimentdescribed in FIG. 1 also. The wedge form could further be used in otherembodiments alone or in combination with some other compensation for theoptical aberrations caused by the plates.

In FIG. 3 a third embodiment according to the invention is shown inwhich like numbers are used to indicate corresponding details. In thisembodiment the aberration compensation is accomplished by thecompensating plate 307. The lens system 301, the prism 302, the focusinglens 304, the reticle 305, the eyepiece 306 and the optical axis AO aresimilar to each other and serve the same purposes.

In this third embodiment transmitter 311, transmitting light of thewavelength λ₁ and the second transmitter 312 transmitting light of thewavelength λ₂, are arranged in the same manner as in embodiment 1 usingdichroic mirrors 313, 314 to bend the light towards the prism 302.

The optical path for λ₁ for the light reflected from the target willessentially be the same as in the first embodiment via the dichroicmirror 321 and 331 towards the receiver 341. The beam λ₂ has a beam pathessentially as in FIG. 1 reflected in mirrors 322 and 332 to thereceiver 342.

In this embodiment a compensating plate 307 with a tilt axis oriented90° with respect to the tilt axis of the other plates is inserted tocompensate for optical aberrations caused in the visible channel by thevisible light having to transverse the plates 321 and 322.

In FIG. 4 a a fourth embodiment according to the invention is shown inwhich like numbers are used to indicate corresponding details. The lenssystem 401, the prism 402, the focusing lens 404, the reticle 405, theeyepiece 406 and the optical axis AO are similar to each other and servethe same purposes.

In this forth embodiment a first transmitter 411, transmitting light ofthe wavelength λ₁ arranged perpendicular to the optical axis OA and thesecond transmitter 412 transmitting light of the wavelength λ₂ arrangedcoaxial to the optical axis OA, using dichroic mirror 414 to bend thelight towards the prism 402.

The optical path for beam λ₂ is reflected by the dichroic mirror 422 inthe direction of a detector 442, the tilt axis of plate 422 in this casebeing oriented perpendicular to the tilt axis of plate 421. Thisrepresents the third method of compensating for optical aberrations.

The optical path for λ₁ for the light reflected from the target willessentially be the same as in the first embodiment via the dichroicmirror 421 and 431 towards the receiver 441. Part of the beam, lighthaving the wavelength λ₁ is reflected in the first tilted plate 421towards a further dichroic plate 431 which reflects the beam towards adetector 441. In this embodiment the tilt axis of the second plate 422is essentially arranged perpendicular to the tilt axis of the firstplate 421, thus serving to compensate for optical aberrations in thevisible channel from the first plate

A top view (detail of FIG. 4 a) is shown in FIG. 4 b. The axis 451 and450 around which the plates are tilted are shown.

In FIG. 5 is shown a fifth embodiment according to the invention. inwhich like numbers are used to indicate corresponding details. The lenssystem 501, the prism 502, the focusing lens 504, the reticle 505, theeyepiece 506 and the optical axis AO are similar to each other and servethe same purposes.

Further in FIG. 5 is shown a combination of a dichroic plate 521 and adichroic prism 522. Beams having wavelengths λ₁ and λ₂, respectively,are transmitted from senders 511 and 512. The optical path for λ₁ forthe light reflected from the target will essentially be the same as inthe first embodiment via the dichroic mirror 521 and 531 towards thereceiver 541. In this case the optical aberrations can be compensated bymaking the plate slightly wedge-shaped, if necessary.

The embodiments described are to be understood as mere examples ofembodiments according to the invention. Because the dichroic plates arerelatively inexpensive as compared with complicated prism systems andalso because the relative compactness that can be accomplished accordingto the invention this contributes to an overall compact and lightweightdesign of apparatuses for tracking and range-finding apparatuses.

1. Electronic distance measuring apparatus for surveying, e.g. formeasuring the distance from the apparatus to an object comprising: a) anobjective lens defining an optical axis (OA), b) at least two sources ofstructured light for transmitting beams (λ₁,λ₂) of separate wavelengthstowards said object, said beams on reflection from said object receivedby the objective lens, (c) at least two receivers arranged outside abeam path as defined by the objective lens, and adapted to receive saidreceived beams (λ₁,λ₂) of structured light, whereat optical meanscomprising at least two dichroic surfaces are each arranged at a tiltangle (α₁, α₂) with respect to said axis (OA), said optical axis (OA)passing through said surfaces, at least one of said dichroic surfacesarranged on a plate, said at least two dichroic surfaces adapted toreflect at least one of said received structured light beams (λ₁,λ₂),respectively, towards said receivers.
 2. Electronic distance measuringapparatus according to claim 1 such that said dichroic surfaces, eachare arranged on separate plates.
 3. Electronic distance measuringapparatus according to claim 1 wherein one of said dichroic surfaces isarranged on a plate and the other of said dichroic surface is arrangedin a prism.
 4. Electronic distance measuring apparatus according toclaim 1 wherein said at least two sources of structured light fortransmitting said beams (λ₁,λ₂) are adapted to transmit said lighttowards a light redirect member arranged adjacent to the objective lens.5. Electronic distance measuring apparatus according to claim 4 whereinsaid light redirect member is a redirecting prism.
 6. Electronicdistance measuring apparatus according to claim 5 wherein saidredirecting prism is attached to the objective lens.
 7. Electronicdistance measuring apparatus according to claim 2 wherein at least onemirror is arranged adjacent to said optical axis (AO) for redirecting atleast one of the received structured light beams (λ₁,λ₂) reflected insaid dichroic surfaces towards said receivers.
 8. Electronic distancemeasuring apparatus according to claim 2 wherein said dichroic platesare wedge-formed, to provide correction for aberration errors. 9.Electronic distance measuring apparatus according to claim 2 wherein acompensating plate is inserted between the focussing lens and adjacentdichroic plate, whereat the tilt direction of said compensating plateand said dichroic plate, said tilt direction defined by an axis in theplane of the plate around which the plate is rotated with respect tosaid optical axis, are chosen such that the tilt directions of the twoplates are arranged such as to be at an angle of approximately 90° withrespect to each other.
 10. Electronic distance measuring apparatusaccording to claim 2 wherein two dichroic plates are arranged such thatthe tilt direction of said dichroic plates, said tilt direction definedby an axis in the plane of the plate around which the respective plateis rotated with respect to said optical axis, are chosen such that thetilt directions of the two plates are arranged to be at an angle ofapproximately 90° with respect to each other.
 11. An electronic distancemeasuring apparatus according to claim 3 wherein at least one mirror isarranged adjacent to said optical axis (AO) for redirecting at least oneof the received structured light beams (λ₁,λ₂) reflected in saiddichroic surfaces towards said receivers.